The above-mentioned fine polymer particles are fine particles of polymer, and these fine particles generally can have a wide range of diameters from several tens of nm to several hundreds of μm. Unlike polymer moldings such as film, fiber, injection moldings, and extrusion moldings, fine polymer particles are used for modification and improvement of various materials by making use of the large specific surface area and the structure of fine particles. Their major uses include modifying agents for cosmetics, additives for toners, rheology modifying agents for paints, agents for medical diagnosis and examination, and additives for moldings such as automobile materials and construction materials. In particular, they have been in wider use in recent years owing to the advantageous fine particle structure of fine polymer particles, as material for rapid prototyping and rapid manufacturing, i.e., techniques to produce custom-made moldings using laser processing techniques.
As such fine polymer particles, furthermore, there are increasing demand in recent years for fine polymer particles that have high heat resistance, high solvent resistance, and uniform particle diameter distribution.
Conventional methods for producing common fine polymer particles can be roughly divided into two types: building-up processes such as emulsion polymerization and top-down processes such as mechanical crushing, melt kneading, dissolution-deposition, and emulsion-precipitation.
The major building-up processes include radical polymerization of vinyl polymers such as emulsion polymerization. Such radical polymerization processes have conventionally been in wide use for production of colloid particles, and they can produce particles from several tens of nm to several μm. Fine particles produced by radical polymerization have conventionally been studied as material; for modifying agents for ABS resin, spacer material for liquid crystal display, and emulsion paints, and thus, the method has been a general means of obtaining fine polymer particles.
The polymers that have been studied for these techniques, however, have been limited to vinyl polymers produced from acrylic, or styrene resin as input material, and they have some problems such as poor heat resistance, i.e., resistance to temperatures up to only about 150° C. (Japanese Unexamined Patent Publication (Kokai) No. 2007-254727, Japanese Examined Patent Publication (Kokoku) No. HEI-4-71081 and Japanese Unexamined Patent Publication (Kokai) No. HEI-7-133328). It is generally possible to produce these fine vinyl polymer particles with a relatively uniform particle diameter distribution in the submicronic range. Their particle diameter distribution tends to become larger, however, when production of particles of 500 nm or more is attempted and, therefore, particle diameter control with a special technique is necessary to obtain a narrow particle diameter distribution. Such special particle diameter control techniques include the dispersion polymerization method that focuses on solubility and the macromonomer method that uses an initiator with special characteristics (Journal of Applied Polymer Science, Vol. 71, 2271-2290 (1990) and Japanese Unexamined Patent Publication (Kokai) No. 2004-149569). These techniques, however, are also limited to vinyl polymers, and difficult to apply to a wide range of polymers.
One of the major top-down processes is the mechanical crushing method, which can be performed easily. With this technique, polymer pellets, etc. are mechanically crushed after being frozen in liquid nitrogen, etc.
However, that technique requires freezing of the polymer. This leads to a larger energy cost, and the resulting crushed particles generally have random shapes. The currently available processes, furthermore, can achieve an average particle diameter only down to a lower limit of about 1 μm, and it is extremely difficult to pulverize polymer material to a smaller size.
The melt kneading method (Japanese Unexamined Patent Publication (Kokai) No. 2005-162840), dissolution-deposition method (Japanese Unexamined Patent Publication (Kokai) No. 2005-054153), the emulsion method, etc., have been developed to provide techniques to produce spherical fine polymer particles.
The melt kneading method described in JP '840 uses an extrusion molding machine. In this production method, a medium component incompatible with the polymer is melt-knead at a high temperature to form a sea-island structure, and then the sea component is dissolved to take out the internal particles. The melt kneading method has some problems. It requires a high temperature, and the resulting fine particles tend to have a wide particle diameter distribution. Furthermore, kneading has to be performed under high-viscosity conditions, and this makes it difficult to largely decrease the average particle diameter.
In the dissolution-deposition described in JP '153, a polymer is heated under the existence of a solvent up to a high temperature to form a polymer solution, which is then cooled to produce fine particles. In that technique, however, the output is limited to the solubility of the polymer in the solvent and, in many cases, it is disadvantageous in terms of productivity.
Generally known for long is the emulsion method in which a polymer or a precursor of a thermosetting polymer is dissolved in an organic solvent, or a polymer is melted, and the resulting solution or melt is combined with water to provide an emulsion which is subsequently processed into polymer particles while maintaining the shape. Based on the emulsion method, Japanese Unexamined Patent Publication (Kokai) No. SHO-63-75038, etc. have proposed a process that comprises dissolution of polyurethane resin in an organic solvent and production of an emulsion in water, and Japanese Unexamined Patent Publication (Kokai) No. HEI-3-168217 proposes a process that comprises dissolution of polymethyl methacrylate in methylene chloride and production of an emulsion in water. To produce fine polymer particles from a precursor of a thermosetting polymer, the emulsification-solidification method is known, in which a polymer precursor is emulsified in a dispersion medium and the subjected to a curing reaction (Japanese Unexamined Patent Publication (Kokai) Nos. HEI-1-158042 and HEI-6-287271).
The emulsion method, however, has the feature that the resulting particles have the same diameter as that of the emulsion particles. This emulsion particle diameter depends on the stirring power, viscosity, and interfacial tension, and the particle diameter distribution is large in most cases (Nyuka•Bunsann Purosesu No Kino To Oyogijutsu (published in 1995 by Science Forum Inc.).
Mechanical crushing and chemical particle production have been known as methods to produce particles aromatic polyethersulfone (hereinafter abbreviated as PES particles).
With respect to mechanical crushing, a process that uses a crushing machine to crush aromatic polyethersulfone (hereinafter abbreviated as PES) into particles with a size of several tens μm has been disclosed (JP '727). As particles are crushed to a smaller diameter down to 50 μm or less, however, the time, cost, etc., required for the crushing extremely increase while the productivity decreases. In addition, the particle diameter distribution will become broad and difficult to control.
With respect to chemical particle production, a process that dissolves PES in N-methyl-2-pyrolidone (NMP), adds ethanol to produce a solution, adds it to a solution prepared by dissolving octylphenoxy polyethoxyethanol in pure water to provide an aqueous dispersion liquid containing particles with a diameter of 1 μm or less has been disclosed (JP '081). The document, however, includes no explicit description about the particle diameter distribution, and there is the problem that the process has to use many types of solvents and need difficult steps. There is a document that discloses a process for producing particles by drying in a liquid, but specific techniques are not shown in its examples, making it difficult to infer its practicability (JP '328). In general, the process for drying in a liquid has the problem that it needs difficult steps and a large cost for removal of solvents which leads a decreased productivity.
It could therefore be helpful to provide a simple method, as compared to the conventional methods, to produce fine polymer particles with a uniform particle diameter distribution, and also to provide a method that can easily produce fine particles of various polymers, including those of highly heat resistant polymers that have been difficult to produce with the conventional methods, and fine polymer particles that are produced therewith.